1. Field of the Invention
The present invention generally relates to a progressive focal lens and eyeglasses using the same, and, more particularly, to a progressive focal lens which provides improved near vision.
2. Description of the Related Art
Reduced resilience of the eye lenses of old people lowers their ability of adjusting focal length. To compensate for this reduced power of accommodation, myopic eyeglasses (presbyopic eyeglasses) are used. When the wearers do not need myopic eyeglasses, the wearers take off the eyeglasses or replace the eyeglasses with hyperopic eyeglasses. Due to the inconvenience of changing between myopic eyeglasses and hyperopic eyeglasses, progressive focal lenses that allow a single pair of eyeglasses to be used for both hyperopic and myopic usages would be advantageous.
As shown in FIG. 1, a typical progressive focal lens 11A has a distance area 13 located at an upper portion of the lens, a reading area 14 located at an lower portion of the lens, and an intermediate area or progressive area 15 positioned between the distance area 13 and the reading area 14. More specifically, the lens 11A has a principal meridional line 12 and a geometric center 01. A distance point 02 is provided on the principal meridional line 12 above the geometric center 01, and a reading point 03 is provided on the principal meridional line 12 below the geometric center 01. The distance area 13 is defined above a first horizontal line L1 that passes through the distance point 02. The distance area 13 has a spherical shape or a near spherical shape. The reading area 14 is defined below a second horizontal line L2 that passes through the reading point 03. The progressive area 15 is defined between the lines L1 and L2.
The progressive area 15 has an aspherical shape and a curvature that gradually changes from the curvature for the distance vision to the curvature for the near vision. The aspherical shape produces distortion. The distortion causes a person wearing the eyeglasses to encounter horizontal distortion when the line of sight is shifted vertically or results in a change in the size ratio of an object viewed when the line of sight is shifted horizontally. When the line of sight passes the progressive area 15, therefore, reeling occurs, causing the wearer to feel uncomfortable. Because astigmatism associated with distortion is present in the progressive area 15, an object being viewed may be blurred.
Japanese Patent Publication No. Hei 1-5682 proposes a progressive focal lens which provides comfortable binocular side vision close to that of a naked eye in consideration of the action of a wearer to turn his head to view the sides with both eyes.
This progressive focal lens will be discussed in detail below. As shown in FIG. 2, a part of the principal meridional line 12 that lies in the progressive area 15 and the reading area 14 is shifted by a predetermined distance toward the nose side as indicated by the broken line 121. The distribution of astigmatism is horizontally asymmetrical about this shifted principal meridional line 121 in the area which lies within 15 mm horizontally toward the nose side and the ear side from the shifted principal meridional line 121. The horizontal asymmetry is indicated by equi-astigmatism curves 21 and 22. Further, an increase in astigmatism toward the ear side in the reading area 14 and the progressive area 15 is slower than an increase in astigmatism on the nose side.
When eyeglasses are used, as shown in FIG. 3, the moving angle .theta.1 of the line of sight on the ear side is greater than the moving angle .theta.2 of the line of sight on the nose side. Due to the slower increase in astigmatism on the ear side in this progressive focal lens, reeling of an object to be viewed at the time of moving the line of sight on the ear side is suppressed. When the wearer views a stationary object 23 through the reading area 14 and the progressive area 15, however, the nose sides of the lenses 11A are used most. In near vision, a wearer frequently sees something in front of the face and between both eyes. In near vision, therefore, it is desirable that astigmatism and distortion in the nose-side area should be small. However, the progressive focal lens 11A has a large change in astigmatism in the nose-side area in near vision. This progressive focal lens 11A therefore causes problems, such as blurring of the object 23, in addition to large distortion and large reeling caused by movement of the line of sight.
To view an object which lies rightward of the right eye or leftward of the left eye at a close distance, it is natural for a person to turn their head to view the object directly. This allows the person to see the object more clearly with both eyes. As the progressive focal lens 11A has the reading area 14 narrower than the distance area 13, it is customary for the wearer of eyeglasses using the progressive focal lenses 11A to see the object in front of the face in near vision. Further, the area of the progressive focal lens 11A that is located on the nose side, rather than the area near the principal meridional line 12 is frequently used in near vision. Thus, the principal meridional line 12 is shifted toward the nose side in association with convergence. In this case, the convergence corresponds to the distance of clear vision (about 30 cm). In real life, people using the progressive focal lenses 11A are apt to see objects lying closer than 30 cm. It is therefore desirable that astigmatism and distortion in the nose-side area, which is frequently used, be smaller.
Japanese Unexamined Patent Publication No. Hei 3-230114 discloses a progressive focal lens which has the principal meridional line 12 horizontally asymmetrical in the reading area 14 and the progressive area 15 and the maximum astigmatism on the ear side in the reading area 14 is equal to that on the nose side, as shown in FIG. 4. In addition, vertical distortion .delta.1 on the ear side in the reading area 14 and the progressive area 15 is smaller than vertical distortion .delta.2 on the nose side as shown in FIGS. 5 and 6. That is, the curvature in the reading area 14 is greater on the nose side than on the ear side. The reason for making distortion on the ear side smaller than that on the nose side is that when the neck is moved right and left, a new image comes from the ear side.
The wearer views the nearby object 23 through the reading area 14 and the progressive area 15 on the nose side of the lens. When the wearer sees the object 23 over the area that extends from the progressive area 15 to the reading area 14, therefore, large distortion occurs in the vertical direction of the image of the object 23. This distortion undesirably increases the asthenopia.
Accordingly, it is an objective of the present invention to provide a progressive focal lens that permits an improved near vision in the nose-side area.